What was Mrs. Weasley Thinking?

[Robert Jones]

No: HPFGUIDX 89976

In OOTP ch. 9 (p. 63  US ed.), when Mrs. Weasley sees that Ron is 
made Prefect she says "That's everyone in the family!" to which Fred 
replies "What are George and I, next door neighbors?

Well, what was Mrs. Weasley thinking?  How could she forget two of 
her children like that?  (I'm giving Molly the benefit of the doubt 
and assuming she is not also forgetting about Ginny — that is, she 
meant "That's everyone in the family so far!")

If JKRis just giving a set up for one of the twin's jokes, it is 
kind of mean — having a mother forget about two of her children 
right in front of them.

But maybe Mrs. Weasley was caught up in the excitement of the moment 
and let something slip?  Maybe Fred and George are not really her 
children?

In PS/SS (ch 6, p. 92), the very first time we meet the Weasleys and 
the very first joke from Fred that we hear has Fred pretending to be 
George and then say "Honestly, woman, you call yourself our 
mother?"  I thought it was just a joke, but maybe it's a clue to 
something.